On Valentine's Day, Real Madrid and Paris Saint-Germain enter the Champions League Thunderdome and there is no exiting -- mentally, at least -- until March 6, the return leg for the knockout round of 16.

Few clashes of this magnitude have been as zero-sum at this stage of the campaign, ever.

For the reigning European champions, it's real simple. This is Real Madrid's lifeline. Fail to pull yourself to safety and the season is over on March 7, with the gut-wrenching thought of no more meaningful football for another six months.

As for PSG, in some ways, the impact could be even greater. They'll still have trophies to play for -- and win -- but their battle is as much legal and financial. And there is no telling what damage defeat could bring, but it'd likely go beyond bruised egos.

Start with Madrid, winners of three of the past four Champions League crowns and their numerical horror show of being 17 points behind arch-rival Barcelona in La Liga, albeit with a largely irrelevant game in hand and on pace for their lowest points total in 11 years. They were also knocked out of the Copa del Rey by Leganes, a club playing in only their second ever top-flight season. Indeed, Real Madrid wouldn't even be facing the juggernaut that is PSG if not for finishing second in the Champions League group stage.

Expectations are routinely high, but also routinely met, at the Bernabeu, which is why when all the wheels come off at the same time this season, the reaction is as much one of wonder as it is of despair. Squeezing as much as you can out of a dynasty and knowing how to manage a cycle is an art form. Madrid felt confident that they had done that, locking up the starters with long-term contracts and re-stocking the bench with young talent. For another season at least, they thought this would be a wind-up-and-go team, so blessed with star power, experience and professionalism that all it would take would be some Galactico-whispering from manager Zinedine Zidane to keep them at the highest level.

But it didn't take much to throw Real Madrid off kilter: an early season goal drought for Cristiano Ronaldo, a few injuries, loss of form in other areas, some youngsters who were a little too green, a dash of bad luck -- nothing decisive on its own, yet all of it forming an ugly mosaic.

An exit in the Champions League would mean no finger left to hide behind. Local media, some of whom are already demanding a deep purge and rebuild, would be left with nothing to do but plan next season. It likely would happen anyway, the difference is that it will happen sooner, it will be public and it will be all-consuming in the world's biggest goldfish bowl. Zidane, of course, won't be safe either: Bernabeu icon status, plus two European Cups, two Club World Cups and a Liga title in 26 months buy you a lot, but not lifetime immunity.

A Champions League run -- heck, even winning three in a row -- won't paper over the cracks. But it will buy the club time for an orderly rebuild.

PSG's issues are slightly different. The Champions League was the hurdle at which they fell last season, after an unreal, acrimonious and highly controversial two legs against Barcelona. They reacted by reloading, which meant keeping faith with manager Unai Emery, who failed deliver the domestic league to PSG for the first time in five years, and pulling out all the stops in the summer.

PSG paid a quarter-billion dollar buyout clause to make Neymar the most expensive player in history, more than doubling the previous record. The fact that Neymar came from Barcelona and left against their will was a massive statement. PSG then picked up the second most-expensive ever, Kylian Mbappe, for good measure. (Forget the loan-plus-buyback shenanigans: Mbappe will cost them a minimum of $190 million, possibly more.)

That's the sort of investment that demands progress. Merely winning the three major domestic trophies, which they're likely to do this year, just like in 2014-15 and 2015-16 seasons, isn't enough.

There's another obvious, elephant in the room. In 2014, PSG breached UEFA's Financial Fair Play regulations as their losses exceeded the allowable amount. Since then, PSG's revenues have grown massively and they've been compliant, but the spending spree of 2017 raises serious questions. Exiting in the Round of 16 versus, say, losing the Champions League finals, means leaving some $30m on the table in prize money. That's before you get into match revenue, commercial revenue and the sponsorship opportunities that a deeper cup run would bring.

Would it be enough to move the needle one way or the other? Truth is, nobody knows right now, not even UEFA's bean-counters. But it's safe to say that winning the Champions League would go a long way towards helping them meet their FFP targets or, at least, come close enough to get a pass. Not to mention that a hypothetical sanction would be much easier to swallow if it came with PSG's first European Cup: which would also be French football's second ever and first in a quarter century.

By the end of the first week in March we could witness the closing credits of Real Madrid's second Galactico Era. Or we could be seeing PSG's grand summer gamble blow up in their faces ... possibly with an FFP sanction around the corner.